Since you're new to ASP.NET Core 2.0, I am not going to explain things in details but I very want to encourage you to go research them and later on master them by yourself.

Regular Form Post

LoginViewModel.cs

namespace DL.SO.Project.Reporting.Accounts
{
    public class LoginViewModel
    {
        [Required]
        public string Username { get; set; }

        [Required]
        [DataType(DataType.Password)]
        public string Password { get; set; }

        [Display(Name = "Remember my login?")]
        public bool RememberMe { get; set; }

        public string ReturnUrl { get; set; }
    }
}

Login.cshtml

@model DL.SO.Project.Reporting.Accounts.LoginViewModel
@{
    ViewBag.Title = "Login";
    Layout = "~/Views/Shared/_CenteredLayout.cshtml";
}

<div class="widget">
    <form asp-area="" asp-controller="account" asp-action="login">
        <input type="hidden" asp-for="ReturnUrl" />

        <div asp-validation-summary="ModelOnly" class="text-danger"></div>

        <div class="form-group">
            <label asp-for="Username" class="required"></label>
            <input type="text" class="form-control" asp-for="Username" autofocus="autofocus" />
            <span class="form-text" asp-validation-for="Username"></span>
        </div>
        <div class="form-group">
            <label asp-for="Password" class="required"></label>
            <input type="password" class="form-control" asp-for="Password" autocomplete="off" />
            <span class="form-text" asp-validation-for="Password"></span>
        </div>
        <div class="form-check">
            <label class="custom-control custom-checkbox">
                <input type="checkbox" class="custom-control-input" asp-for="RememberMe" />
                <span class="custom-control-indicator"></span>
                <span class="custom-control-description">
                    @Html.DisplayNameFor(m => m.RememberMe)
                </span>
            </label>
        </div>
        <button type="submit" class="btn btn-primary btn-block">Login</button>
    </form>
</div>

See the asp-controller on the form element, and those asp- attributes on inputs? They're built-in tag helpers from ASP.NET Core.

For example, with asp- attributes on form element, it will create a form with POST method attribute and action attribute pointing to /account/login. And hopefully you have an Account controller Login method to handle that request.

Optional: for client side validation to work, you need jquery-validation and jquery-validation-unobtrusive packages.

Ajax Form Post

To use AJAX for the form post, there are some additional steps beside building forms like above.

• Need to install package jquery-ajax-unobtrusive.
• Need to specify ajax callbacks using data- on the form element.
• Need to define the callback functions.

For example,

<form asp-area="" asp-controller="account", asp-action="login"
    data-ajax="true" data-ajax-method="post" data-ajax-begin="onFormBegin"
    data-ajax-complete="onFormComplete">
...
</form>

For a complete list of Unobtrusive ajax helpers, please see here: https://dotnetthoughts.net/jquery-unobtrusive-ajax-helpers-in-aspnet-core/

And then you need to define the callback functions.

@section scripts {
    <script type="text/javascripts">
        $(function() {
            window.onFormBegin = function() {
                ...
            };

            window.onFormComplete = function(request, status) {
                ...

                // if the status is not valid
                //     keep displaying the form?
                // just an idea...

                ...
            };
        });
    </script>
}

My Taste

This is just my opinion. Whenever I deal with account login and other security related stuff, I tend to use regular form post instead of ajax. I don't want anything fancy there, because when something goes wrong, I want the server to give the user a fresh page, or redirect user to somewhere directly and immediately, instead of keeping the page that has invalid information and using ajax to communicate.

Again this is just my personal taste.